Is there a reason why /var/log/lastlog is a huge sparse file (1.1TB)?

I have read some question, that ask advice how to rsync sparse files efficiently mentioning the files /var/log/lastlog and /var/log/faillog. Indeed I myself have stumpled over those files being an "issue" as their being backup via rsync turns them to become "unsparse". What I hence wonder is, what is the need/backgrounding motivation to have those files as sparse, huge files (in my case it was 1.1TB)? Also in relationship to this a follow up: Since I was assuming them to be logfiles I do not care about excesively I truncated those files, did I corrupt anything with truncating those files ?